The invention relates to a display device for use in reflection, provided with a layer of liquid crystalline material between a first transparent supporting plate having at least one transparent first drive electrode and a second supporting plate having at least one controllable picture electrode, said layer of liquid crystalline material being switchable between two states via electric voltages on the picture electrodes, while, dependent on the applied electric voltage, the device traverses a range of continuously decreasing or increasing values of reflection between a first state, in which the molecules of the liquid crystalline material have a first direction of orientation which is substantially parallel to the supporting plates or extend at a small angle to the supporting plates, the direction of orientation, viewed across the thickness of the layer of liquid crystalline material, having a rotation or twist, and a second state, in which the molecules of the liquid crystalline material have a second direction of orientation which is substantially perpendicular to the first direction of orientation, the difference in optical path length difference between the ordinary and extraordinary wave between the two states being substantially 1/2 .lambda..sub.0 after reflection for an incident beam having a central wavelength of .lambda..sub.0.
The invention also relates to a projection display device comprising such a display device.
A device of the type described in the opening paragraph is disclosed in EP 0,377,757. In this Application it has been described how an optimum reflection is obtained for two specific twist angles (63.degree. and 193.degree.) for a reflective liquid crystal display device having a polarizer which is parallel to the director of the liquid crystal material at the area of the front face, with associated optimal values for the optical path length difference d..DELTA.n (d: thickness of the liquid crystal layer, .DELTA.n: difference in refractive index between the ordinary and the extraordinary wave). Notably for the angle of 63.degree. this leads to a low value of d..DELTA.n so that the display device will have a small thickness (up to approximately 2 .mu.m) in practice. Moreover, the device is optimized for one wavelength when used between parallel polarizers. As a result the light incident on the mirror is not always circularly polarized. In a wavelength range (500-600 nm) to be used for projection display there is thus a variation of the transmission in the dark state (at low voltages) up to about 4%, which considerably reduces the contrast.
Moreover, at high voltages, peripheral effects occur between the director at the rear wall and wave components of the wave to be reflected, thus introducing an additional birefringence.